Mark Landler

SEOUL, South Korea: President Barack Obama took North Korea’s untested new leader, Kim Jong Un, to task Monday, demanding that China curb his recent behavior and declaring that South Korea’s success will inevitably triumph over the failure and isolation of the North.

In a 90-minute meeting with China’s president, Hu Jintao, before a nuclear security summit, Obama pressed the Chinese leader to use his influence over Pyongyang to stop it from proceeding with a satellite launch next month, according to a White House official.

Obama was clearly stung by North Korea’s announcement of the impending satellite launch, which the United States said would be a breach of its international obligations, and which came only 17 days after the Obama administration had tentatively agreed to send North Korea desperately needed food aid.

But the president, the senior official said, was also latching on to the change in leadership in Pyongyang to try to break what Obama called a long cycle of provocations that were rewarded by countries eager to ease tensions with an erratic nuclear state.

“We need to have a serious conversation with the North Koreans where they understand that we’re going to do things differently in the future,” said the senior official, the deputy national security adviser, Benjamin J. Rhodes.

Hu told the president that China shared his concern about the satellite launch and had conveyed that to North Korea, Rhodes said. But Hu appeared surprised by the vehemence of Obama’s message, according to senior officials.

In that meeting, as well as an earlier one with President Dmitry Medvedev of Russia, Obama was trying to budge two countries that have been the greatest impediments to more aggressive action against North Korea, Iran and Syria.

Domestic politics surfaced unexpectedly in the president’s meeting with Medvedev, after an open microphone picked up Obama telling the Russian president that he would have more flexibility on issues like missile defense after the election in November.

“I understand,” Medvedev replied, leaning forward and speaking softly. “I transmit this information to Vladimir,” he said, referring to Russia’s newly elected president, Vladimir Putin.

Asked about the exchange, Rhodes said the president was committed to building the missile-defense system, despite Russia’s objections, and that he was merely acknowledging that an election year was not the best time to negotiate.

Obama’s meetings had a valedictory feel. It was Medvedev’s last session with him before being replaced by Putin. And it may be one of the last times he meets with President Lee Myung-bak of South Korea, whose term expires next year.

Still, the president’s mind seemed most fixed on a young man he has never met: Kim Jong Un, the new leader of North Korea, who ascended following the death of his father, Kim Jong Il, in December.

Speaking to 400 students at a foreign-affairs university, Obama challenged Kim Jong Un and his subordinates to give up their belligerent behavior and “have the courage to pursue peace and give a better life to the people of North Korea.”